General anesthesia is known to alter cardiopulmonary and metabolic parameters which affect tissue oxygenation. We have examined the changes of cerebral tissue pO2 and physiological parameters after injection of anesthetics in rats using EPR oximetry. Two dosages of ketamine/xylazine (100/10mg/kg and 50/5mg/kg i.m.) were investigated as a first stage for this study. Animals were allowed to breathe spontaneously. Cerebral tissue pO2 in both groups was almost constant until awakening ( 13.6, 11.9, 12.1, 12.4, 13.3, 13.6 mmHg at 30, 60, 90, 120,150, 180 min after injection respectively in high dosage group; 16.5, 15.4, 16.4 mmHg at 30, 60, 90 min in low dosage group). These pO2 values were extremely low compared with the value in an unanesthetized state (30~35mmHg) which we previously measured. The change of mean arterial pressure corresponded with that of cerebral pO2 . These data indicate that ketamine/xylazine anesthesia causes a substantial decrease of cerebral pO2 and its effect still remains at the awakening period. The experimental approach was extended to include the use of ventilation and various potential pressures of gas. Data then were obtained in foru different anesthetics: isoflurane, halothan, pentobarbital and ketamine/xylazine. See Highlight #6 for details.